The optical fiber geometry is unique, combining micrometer to nanometer scale transverse dimensions with kilometer scale axial dimensions (Russell, P. Photonic Crystal Fibers: A Historical Account. IEEE Lasers and Electro-Optics Soc. 21, 2007). Arranging materials in such geometries offers interesting opportunities to exploit very weak effects in materials due to the extremely long interaction lengths of photons and electrons. Silica-based optical fiber waveguides are ubiquitous in transmitting light over long distances for the flexible delivery of optical power in technologies such as communication, surgery, and sensing due to their high purity, mechanical strength, and uniform, smooth structure (Gambling, W. A. The Rise and Rise of Optical Fibers. IEEE Journal on Selected Topics of Quantum Electronics 6, 2000). Silica optical fiber lasers have emerged as compact, rugged, stable, and very high power sources of both continuous wave and ultrafast near-infrared light and are preferred for many applications. These advantages could be extended into other wavelength domains if fiber lasers could be fabricated out of a range of materials beyond silica and other glasses. In contrast to silica, many II-VI compound semiconductors have transmission windows that extend much farther into the IR making them very useful for applications such as chemical and thermal sensing, infrared countermeasures, stand-off detection of explosives, and nonlinear optics (Méndez, J. A. & Morse, T. F. Specialty Optical Fibers Handbook, Elsevier, 2006; Harrington, J. A. Infrared Fibers and Their Applications, SPIE, 2004). However, fabricating optical fibers out of crystalline laser gain media has proven to be challenging using conventional fiber drawing techniques.
Semiconductor optical gain media such as Cr2+-doped zinc selenide (Cr2+:ZnSe) and related crystalline transition metal-doped II-VI chalcogenides in particular are very attractive for efficient infrared lasers tunable in the technologically important 2 to 5 μm mid-infrared region of the spectrum. Cr2+:ZnSe lasers share many desirable characteristics with Ti-Sapphire lasers, a preferred source for a very wide range of continuous wave (CW) and pulsed applications, but emit in the technologically important 2-3 μm mid-infrared region of the spectrum rather than in the red and near-infrared. In particular, Cr2+ does not exhibit any excited state spin allowed transitions and the matrix ZnSe has a low phonon frequency of 250 cm−1, which allows for the highest quantum yield known for the 2-3 μm mid-infrared region, near continuous vibronic emission, and room temperature operation (Kueck, in International Conference on Lasers, Applications, and Technologies 2002, Vol. 57, SPIE, Moscow 2003). Cr2+:ZnSe also exhibits the broadest gain known, making it potentially very attractive for ultrafast applications. Lasers operating in the 2-3 μm wavelength range of Cr2+:ZnSe and the 3.7-5.1 μm wavelength range of the closely related Fe2+:ZnSe materials are sought after for chemical detection, biomedicine, infrared countermeasures, and astronomical applications. High power ultrafast Cr2+:ZnSe lasers could enable scaling of coherent high harmonically generated sources further into the soft x-ray regime, as the cutoff frequency varies with the inverse square of the pump frequency. Additionally, changing the crystalline lattice can alter the crystal field splitting to allow for more tunability (e.g. 1.9-2.8 μm in Cr2+:ZnS; Moskalev, et al., Optics express 17, 2048-2056, 2009) and several other useful transition metal chalcogenide gain media are known.
However, Cr2+:ZnSe based bulk lasers are plagued by thermal effects due to their large thermo-optic coefficient (dn/dT=70×10−6 K−1 for ZnSe; Sorokina, Optical Materials 26, 395, 2004). This gives rise to thermal lensing, which has limited the power of CW lasers that use static bulk gain media to 14 W (Berry, et al., Optics express 18, 15062-15072, 2010; Schepler, et al., IEEE Journal of Selected Topics on Quantum Electronics 11, 2005). Configurations that require movement of the gain media in a high Q cavity to ameliorate thermal lensing are awkward for applications such as those on aircraft and many more. High surface area to volume ratio geometries that facilitate heat removal are therefore of interest for power scaling Cr2+:ZnSe lasers up to the levels needed for many applications.
Optical fibers in particular are less susceptible to thermal lensing effects because of their long, small diameter cores that radiate heat much more effectively than bulk optics. Additionally, thermo-optic induced changes in refractive index do not alter their light guiding properties significantly because the mode of the laser is determined by the mode structure of the waveguide. Heat arising from light absorption and/or quantum defects in fiber laser gain media is radiated equally well over 360 degrees of cross sectional angle; the large surface area-to-volume ratio and long length of the fiber also facilitates management of this heat. The long length of an optical fiber allows for lower concentrations of active ions to be used, while still efficiently absorbing all of the pump light, which reduces the thermal load per unit length on the fiber. Owing to these effects, many of the most powerful commercially available lasers are fiber lasers (Richardson and Nilsson, Opt. Soc. Am. B 27, 63, 2010). In contrast, planar ZnSe waveguides fabricated by a variety of methods are also in general less susceptible to thermal lensing, but lack the circular cross sections favorable to uniform heat radiation and polarization independent guidance. In general, fibers are also noted for higher power handling capabilities in comparison with planar waveguides and can be coupled much more easily to other fibers and fiber devices, thus enabling all-fiber optoelectronics. Encapsulating the ZnSe fiber core into a flexible, strong silica fiber cladding makes it less susceptible to chemical, mechanical, or thermal degradation. Hybrid physical-chemical vapor deposition (HPCVD) is a scalable process that can be used to fabricate many semiconductor fibers with lengths from centimeters to tens of meters in parallel. Competing approaches such as pedestal growth are not viable for chalcogenides such as ZnSe and are not as scalable as fiber drawing or HPCVD.
Cr2+:ZnSe fiber lasers have been proposed to overcome the previously mentioned difficulties (Martyshkin, et al., Optics Letters 2011, 36; J. B. McKay, in Engineering, Air Force Institute of Technology, Dayton, Ohio 2003) but incongruent vaporization of the chalcogen has precluded drawing them at high temperatures. Fabricating optical fibers out of crystalline semiconductor materials using conventional drawing methods poses a significant challenge due to the thermal, chemical, and mechanical materials mismatches of their materials properties with silica. Although some III-V compound semiconductors, such as InSb, can be drawn into optical fibers (Ballato, et al., Optics express 18, 2010), the II-VI semiconductors appear to be among the most challenging materials to form into the optical fiber geometry due to their high vapor pressures and incongruent melting points.
Thus, there is a continuing need in the art for novel materials for fiber optics lasers, in particular transition metal doped semiconductors with uniform doping concentration. There is also a continuing need in the art for novel optic fibers, and fiber optics lasers, in particular Cr2+:ZnSe lasers. There is a continuing need in the art for novel methods for fabricating these materials and optic fibers. The present invention addresses these continuing needs in the art.